The main objective of the present project is to determine the inner workings of the mechanism responsible for the fluid pumping activity of the corneal endothelium. It is also intended to adapt a technique presently employed in vitro so that the electrical resistance of the endothelium can be measured in excised intact eyes. This may provide an objective means of assessing the condition of the endothelial layer in donor material. The rates of fluid pumping and the time transients of these rates induced by different experimental conditions (varying concentrations of Na, K, HCO3, H and other substituting cations and anions, presence of drugs, inhibitors and varied metabolic substrates) will be determined with a sensitive micromethod. The short-circuit current, potential difference, resistance and acidifying capacity of the endothelium will be correlated to the rates of fluid pumping under the different experimental conditions above. The intracellular electrical potentials and ionic concentrations (of Na, K and Cl) will be determined with conventional and ion-sensitive microelectrodes and will be correlated with the transport activity. It is hoped that these studies will contribute to clarify the static and dynamic characteristics of the endothelial transport processes, and to establish the precise way in which the active and passive endothelial mechanisms interplay so as to keep the cornea in its relatively dehydrated and transparent normal condition.